[heap] Notify GC on potentially unsafe object layout changes.

src/objects-inl.h

Index: src/objects-inl.h
diff --git a/src/objects-inl.h b/src/objects-inl.h
index c0ac8d1ee46349027be11cbdf0ef54da4a582d14..50128a25ecc8b3e850960997428f4332c1957150 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -1468,10 +1468,13 @@ Map* HeapObject::map() const {
 
 void HeapObject::set_map(Map* value) {
   set_map_word(MapWord::FromMap(value));
-  if (value != NULL) {
+  if (value != nullptr) {
     // TODO(1600) We are passing NULL as a slot because maps can never be on
     // evacuation candidate.
-    value->GetHeap()->incremental_marking()->RecordWrite(this, NULL, value);
+    value->GetHeap()->incremental_marking()->RecordWrite(this, nullptr, value);
+#ifdef VERIFY_HEAP
+    value->GetHeap()->VerifyObjectLayoutChange(this, value);
+#endif
   }
 }
 
@@ -1483,10 +1486,13 @@ Map* HeapObject::synchronized_map() {
 
 void HeapObject::synchronized_set_map(Map* value) {
   synchronized_set_map_word(MapWord::FromMap(value));
-  if (value != NULL) {
+  if (value != nullptr) {
     // TODO(1600) We are passing NULL as a slot because maps can never be on
     // evacuation candidate.
-    value->GetHeap()->incremental_marking()->RecordWrite(this, NULL, value);
+    value->GetHeap()->incremental_marking()->RecordWrite(this, nullptr, value);
+#ifdef VERIFY_HEAP
+    value->GetHeap()->VerifyObjectLayoutChange(this, value);
+#endif
   }
 }